Still, among energy experts, biomass has some strong supporters, and for good reason: Right now, virtually all the renewable-energy power in Florida comes from biomass, including three plants in Miami-Dade and Broward.

What's more, it's cheap — cheaper in some instances even than coal, which is generally considered the nation's least expensive way of producing electricity but is also the biggest producer of greenhouse gases that scientists say are heating up the globe.

"We're very strong supporters of biomass," says Stephen Smith, head of the Southern Alliance for Clean Energy. "In the short run, it will be a real workhorse." But he adds: "There are various shades of green in biomass. Some is better than others."

As policymakers search for alternatives to fossil fuels that threaten to submerge South Florida under the sea, biomass has emerged as a leading possibility, much more plausible than wind in the state, but it comes with strong pluses and minuses.

Big business has gotten involved. Leading biomass producers — including the multimillionaire Fanjul family with an electric plant burning sugar-cane leftovers — have joined the push to require utilities to use more renewables and pay proper rates for them, which would mean the businesses could get decent revenue by expanding operations.

Still, many have concerns. "Not all biomass is created equal," says Gerald Karnas of the Environmental Defense Fund.

Some environmentalists believe garbage is not as green as, say, wood chips. Others worry that arundo donax, a towering grass proposed by some as a biomass fuel, might spread unintentionally to many areas, including the Everglades, as have other non-native plants.

What's more, major companies in North Florida that use wood products are concerned that state subsidies for renewable energy could drive up the prices of timber that are used for everything from paper to fat-free ice cream.

Even so, research prepared for the Public Service Commission is showing that biomass and solar are the two top practical renewables in Florida.

"The bottom line is that Florida is well positioned for growth in biomass," says Sean Stafford, a lobbyist for Florida Crystals, the Fanjul company. He points out that biomass does not have the "volatile price structure" associated with natural gas, the No. 1 energy source of Florida Power & Light.

What follows is a primer on biomass.

Some biomass — sugar-cane waste, wood chips — is generally considered carbon neutral. As cane and trees grow, they soak up carbon dioxide, the major greenhouse gas. That gas is released when the biomass is burned, meaning they produce energy without contributing to climate change.

Garbage is another story. "In Florida, biomass has a a very broad definition, according to the state legislature," says Karnas. "In some states, power from municipal solid waste wouldn't be considered a renewable."

The reason: Experts dispute how carbon neutral garbage is. "It depends what's in it," says Smith of the Southern Alliance. "If it's mostly yard clippings and paper, that could mean very little carbon. If there's a lot of plastic, that's made by a fossil fuel, meaning it's releasing quite a bit of carbon."

What biomass is not is food. "No one is talking about using food for power," says John Bonitz of the Southern Alliance. "This is not corn and ethanol."

A growing number of critics are speaking out against subsidies for ethanol, saying they've raised food prices and contributed to food shortages in some parts of the world. "Food into fuel is clearly a dumb idea," says Lester Lave, an energy expert at Carnegie Mellon University in Pittsburgh.

In Florida, biomass already produces 1,100 megawatts of power, according a recent study by Navigant Consultants. About half of that comes from 11 waste-to-energy plants that process local garbage and trash.

The other half comes from the leftovers in the timber and sugar-cane industries, which use the power first for their own needs, then sell the remainder to utilities. The biggest plant is run by Florida Crysta1s near South Bay, producing 140 megawatts of power from bagasse, the term for cane waste after the sugar is squeezed out, and from Miami-Dade yard trash.

Florida Power & Light, the state's largest utility, reports that in 2007 it purchased 1.5 million megawatt-hours from biomass plants totaling 303 megawatts of capacity — a tiny fraction of the 25 million megawatts that the utility uses.

Around the nation, biomass plants have a long, well established history. More than half — 53 percent — of all renewables nationwide in 2007 came from biomass, according to the Energy Information Administration.

Biomass advocates point out that this waste produces baseload power — meaning it can fuel plants around the clock — while solar power usually operates about 20 to 25 percent of the time.

In Burlington, Vt., the 50-megawatt McNeil power station has been operating successfully since 1984, using mostly left-over branches, leaves and stumps generated by people harvesting firewood or lumber.

The price for McNeil power fluctuates depending on alternative sources. "There were some times when oil/gas was very cheap when I'm sure our owners were thinking maybe we should have done something else," wrote Irving in an e-mail. But lately, the utility has been getting 10 or 12 cents/kWh, plus a three-cent bonus from Connecticut for providing clean energy. "They're pretty happy now," Irving says of the owners.

What's more, much of the plant's costs are in labor — harvesting the wood waste and getting it to the plant. "So that's economic development, compared to sending all your money to the Middle East."

Meanwhile, Georgia Power proposes changing its 96-megawatt Mitchell Plant from burning coal to biomass. This would not only eliminate a source of the worst emitter of greenhouse gases but would also reduce fuel costs by 30 percent and operating-maintenance costs by 13 percent over the life of the plant, according to spokesman Jeff Wilson.

Most of the wood fuel would come from sources considered unusable by timber companies, Georgia Power says. The switch to biomass is estimated to create 50 to 75 new jobs.

In Florida, Biomass Gas & Electric has deals to build three plants, including a 42-megawatt generator in Tallahassee. BG&E spokesman Keith McDermott says the contract will pay BG&E 7.2 cents/kWh. "Obviously we know we can make the economics work. We're in the business to make money."

The Navigant study reported most types of biomass power's present costs are one-tenth to one-third of solar power's. Even in 2020, assuming major technical improvements for solar, the study found that in one likely scenario, solar will be a viable power source at about 23 cents/kWh, while much of biomass will be at 0.82 to 12 cents/kWh.

Still, Florida biomass producers complain they're not getting paid fairly. Florida Crystals and Covanta Energy, which converts garbage to power, say they often get only 6 or 7 cents/kWh from utilities. FPL reports that so far in 2008 it has paid about 4 cents/kWh for electricity produced by biomass resources — about a third of what its customers pay the utility.

These rates are generally based on "avoided cost of electricity," meaning a wholesale price that a utility says it avoids by buying alternative power.

"The utilities can low-ball us, and there's nothing that we can do about it," says Florida Crystals spokesman Gaston Cantens.

FPL spokesman Mayco Villafana says if the utility paid the biomass producers more, its customers would have to pay more. "In Florida, the rules are written to protect customers by ensuring that utilities don't overpay for the electricity they buy. Eliminating those rules would allow biomass producers to charge whatever they wanted with no protections for customers."

"The system isn't fair," says Joseph Treshler of Covanta Energy, which runs a waste-to-electricity plant in Hillsborough County. "The Legislature gives the utility full-cost recovery for constructing a renewable plant," meaning the utility has no risk and gets all the profit. "There is no incentive for them to look outside. They squeeze the independent."